Something is not in its position if there is no gap at TDC. Double check your manual.:
- Are the intake and exhaust cams in the proper place (not reversed)? They are marked IN and EX. Not trying to insult you, but just looking for clues.
- Are you sure the cams are at the proper position when the flywheel is at TDC?
- Are the cams aligned with the mating surface of the head as it is specified?
- Are there the proper number of cam chain links between the marks on the cams?